Black flecks on negative

I developed a roll of Neopan 400 in Xtol and have noticed black flecks on a couple of the frames. Looking through the negative, the flecks appear completely opaque. But, if I bounce light off the emulsion side, the flecks appear silvery.

What could be the cause?

Sounds like crud in the water. If you gently drag your finger over the flecks, can you feel them?
Are you using filtered water?

No I can't feel them. I've rewashed the neg -- which is otherwise impeccably clean -- but the flecks remain.

I have occasionally had tiny flecks of what appears to the naked eye to be metallic silver on the emulsion side of a film. It happens rarely, and when it does is only ever one or two flecks on a whole film. The flecks are completely opaque, in that you end up with a white spot on your print. But if you look closely, the flecks are bright silver and embedded in the emulsion. I find you can feel these little particles, and if removed they invariably leave a small hole or defect in the emulsion. Don't know exactly what causes them, but have always assumed that it was some sort of local silver build-up during the development process. Maybe the chemists on this site can shed further light...

I have had dark metallic flecks show up in used fixer used for film (I assume that I was seeing metallic silver). Naturally, I simply discarded the fixer, but it did make me wonder what conditions would cause this to happen and whether it might be a good precaution to filter fixer as part of my usual routine (in case the flecks were not readily apparent). So far the problem hasn't repeated itself.

I have seen fixer and developer problems cause this, but also have seen it as an emulsion defect caused by an error in formulation. It is fairly common in home-made emulsions if you don't take good care of what you are doing. I would therefore put the "emulsion defect" source at the bottom of the list and say either developer or fixer problems.

You didn't say what developer or fixer you used.

PE

FWIW, I had problems like this for a month or so about 2 or 3 years ago. Oddly, these problems occurred in both B&W and color films, so there weren't many commonalities in the chemicals and other things I was using -- basically just the water and developing tank. Then the problems went away. I put it down to a temporary problem in my tap water, but I really can't be 100% positive of that; it could have been contamination that somehow got into my bottles or developing tank.

PE - It hasn't happened to me for a while, but in my case almost certainly would have been DD-X (possibly Rodinal) and Hypam.

Well, to be more specific, if the developer or fixer causes the problem it is used solution with silver salts that have accumulated in them through use. Fresh solutions (except for bad fixer) rarely cause the problem, if ever.

Iron flecks or Iron oxide flecks in the water are often a source of this as well.

PE

I was using Iflord Rapid Fix. While not fresh, it hadn't had many films run through it.

Perhaps the problem lies with a mucky storage bottle.

Might be that your fixer is bad then. IDK. Your description sounds like silver deposited in / on the emulsion in a very fine colloid or sheet.

PE